The simplest one is to investigate the effect of light on plants. Get several small pots (if your mom or dad is a gardener they will know where to get some) and some potting soil to each. Get some seeds from a nursery (or beans) Ask them what will grow quickly. Place one seed in each pot and measure the amount of water you give them, so they each get the same. When the shoots start to appear, place one where it will get natural sunlight, one where you can give it artificial light from a lamp, and place the other one where it is dark. Make sure you water them at the same time, with the same amount of water and record your results (you can remove the one from the dark briefly so you can observe and measure it)